Calcium is an essential component in neurons which is involved in a variety of cellular processes ranging from neurotransmitter release to neuronal death. Therefore, fluctuations in the level of cytoplasmic free calcium are expected to play a major physiological and biochemical role. One source of calcium fluctuation in excitable cells is through voltage- dependent calcium channels. In the catfish retina, second order horizontal cells contain a sustained voltage-activated calcium channel which has been hypothesized to have a physiological role in retinal processing. However, little is known about the mechanisms of regulation and modulation of calcium through these channels. An important way in which many neurons modulate their voltage-dependent channels is through the action of second messenger systems. Therefore, this proposal is designed to determine the role of second messenger systems on calcium channel activity in cultured, isolated catfish horizontal cells. The specific aims are: (1) to identify the second messenger systems underlying the modulation of voltage-dependent calcium conductance; (2) to characterize the regulation of these second messenger systems in catfish horizontal cells as they relate to modulation of the voltage- dependent calcium current; and (3) to determine the interactions between multiple second messenger pathways in their modulating role of calcium channel activity. These specific aims will be carried out using a multidisciplinary approach combining whole-cell voltage clamp recordings, excised and on-cell patch techniques for single channel analysis and fura-2 imaging techniques. Results obtained from these studies will contribute greatly to our understanding of the role of calcium in the horizontal cells as well as other neurons.